The AquaticLife 16.5 inch, 10K 18 Watt T5 HO Fluorescent Lamps are 16.5 inches from end of pin to end of pin (for 18 inch fixtures/retrofits).
The 10K lamps are typically used in tanks with corals, but can also be used with fish only tanks, and freshwater tanks with live plants. When used with corals half your lamps should be 10K and half your lamps should be Actinic.
The AquaticLife T5 bulbs will work in any T5 fixture, as long as you are using bulbs of the same wattage as those currently in your fixture. T5 bulbs come in two categories, HO bulbs (18W, 24W, 31W, 39W or 54W) and NOs (9W, 14W, 21W or 28W). T5s are 5/8" or .625" in diameter.
